1. Field of the Invention
This invention relates generally to injection molding and more particularly to a nozzle tip seated in the front end of an injection molding nozzle to provide molten plastic to a mold gate.
2. Related Art
Hot runner nozzle tips may include single or multiple outlet channels for feeding molten resin to a mold cavity in an injection molding system. The hot runner nozzle tips are generally spaced from an injection gate leading to the mold cavity. The spacing is necessary to avoid direct contact between the cooled mold cavity by the heated nozzle tip in order to prevent the nozzle tip from rapidly cooling. Rapid cooling of the nozzle would cause the resin to freeze up and render the nozzle inoperable. In addition, if the nozzle tip is heated from a cold condition to its operating temperature, generally 350°-450° F. space must be provided for thermal expansion, which results in the nozzle and the nozzle tip growing in length.
Conventional hot tip designs usually provide a bubble or resin space surrounding the tip that fills with resin. This resin layer is considered beneficial in that the resin in the bubble has poor thermal conductivity compared to steel and effectively insulates the hot tip from the cooled mold cavity. The amount of resin resulting in the resin layer depends on various factors such as the temperature of the tip, the temperature of the cavity, and the type of plastic resins being processed. However, the presence of resin within the bubble around the nozzle tip results in high pressure drops between the tip area and the mold cavity.
Further, the bubble can result in “hang up” areas around the nozzle tip where resin stays that can narrow the flow path of the melt around the nozzle tip and that can make color or material changing of the resin problematic. Due to these undesirable results, it often becomes necessary to purge the nozzle tip of such resin between drops and/or between material or color changes which can be time consuming and wasteful of the resin. One way in which this is done is to dismantle and clean the nozzle assembly to remove all traces of the previous resin. This can be very time consuming and expensive due to lost production time.
In order to keep melt flowing through a nozzle tip and to effectuate satisfactory color change, one attempted solution is to provide a nozzle tip with an element that induces a spiral flow pattern. Another attempted solution to keep melt flowing through a nozzle tip and to effectuate satisfactory color change is to provide nozzle tips with mixing elements to improve the homogeneity of the resin flowing through the nozzle tips. However, these solutions are not entirely effective in overcoming the foregoing problem and/or promoting rapid color changing.
It is therefore desirable to provide a simple and effective nozzle tip design that does not reduce but improves the melt flow path around a nozzle tip. A nozzle tip according to the present invention achieves this result by incorporating a nozzle tip geometry that increases the surface area of the tip portion to allow for greater heat transfer to the plastic resin in the tip area of the hot runner nozzle. Such a nozzle tip also improves the heat profile, reduces pressure drops, and enhances material flushing at the nozzle tip.